In recent years, a portable radio communication device having an audio data reproducing function and a radio communication function, which includes a mobile phone having a music reproducing function, a music player such as WALK MAN®, a portable game machine such as PlayStation Portable® for example, has been widely used. Such a radio communication device can perform radio communication by a communication mode called an infrastructure mode in which radio communication is performed via a wireless LAN access point (wireless Local Area Network access point) or a communication mode called an ad hoc mode in which radio communication devices directly communicate by radio without the wireless LAN access point. Thus, the radio communication device can access to a server on the Internet via the wireless LAN access point and download audio data (music data, for example) stored in the server or can obtain or exchange audio data by directly transmitting/receiving audio data to/from other radio communication device.
Here, it is known that, in an ideal space, a field intensity of a radio signal transmitted from the radio communication device or the wireless LAN access point varies inversely with the square or the cube of the distance in a vicinity of the radio communication device serving as a signal transmission source and varies inversely with the distance out of the vicinity of the radio communication device. The characteristic of the field intensity of the radio signal is effective with a wireless LAN (Local Area Network) standard such as IEEE 802.11b and IEEE 802.11g.
In this point of view, a technology for estimating a distance between radio communication devices using the characteristic of a field intensity of a radio signal has been developed. The technology for estimating the distance between radio communication devices using the characteristic of a field intensity of a radio signal is seen in Patent Literature 1, for example.    Patent Literature 1: JP 2006-300918 (A)